1. Field of the Invention
The present invention relates to a current lead for electrically connecting a superconducting magnet cooled to a very low temperature to a power supply kept at room temperature.
2. Description of the Related Art
The most important feature of superconductivity is that a large current can flow without any loss. A representative application of superconductivity is a superconducting magnet in a persistent current mode. The superconducting magnet requires current leads for supplying a current from a power supply kept at room temperature to the superconducting magnet kept at very low temperature by liquid helium. Only when the super-conducting magnet is magnetized to a persistent current mode and demagnetized from the persistent current mode, a current flows in the current leads. Therefore, if magnetization and demagnetization are performed once a day, a period for supplying a current to the current lead is several minutes to one hour a day, and the current is not supplied to the current leads for a large part of a day. Since heat is transmitted from a high-temperature side to a very-low-temperature side through the current leads by its thermal conduction in an ON time, the current leads serves as a thermal load to the low temperature end.
In order to reduce the thermal load and effectively drive a superconducting magnet in a persistent current mode, the following two methods are employed.
According to the first method, current leads are formed to be demountable, and the current lead is detached in an OFF time. With this method, an amount of thermal conduction from the current lead in an OFF time can be largely reduced.
According to the second method, stability of the current leads in an ON time is considered. The dimension of the current lead is planned so that a thermal load to very low temperature is minimized in consideration of the current lead in an OFF time. At the same time, the current leads are cooled so that an increase in temperature of the current lead falls within a stable range in an ON time. That is, since an amount of thermal penetration in an OFF time is in proportion to A/L (A: sectional area, L: overall length) of the current lead, the dimension of the current lead is planned so that the value of A/L is minimized. In addition, a current lead conductor is arranged in a cooling tube, and cooling gas is forcibly circulated in the tube to cool the current lead in an ON time. This method is effectively used in a case wherein a superconducting magnet is frequently magnetized to a persistent current mode and demagnetized from the persistent current mode.
In the former method, impurity gas is possibly supplied to a connecting portion between the current lead and the superconducting magnet. When the impurity gas is supplied to the portion, reliability of the operation of the current lead is degraded. Further, when the current lead is detached, the superconducting magnet may not be forcibly demagnetized in a state of emergency. For this reason, this method cannot be employed to all systems.
In the latter method, cumbersome operations such as opening/closing operations of a valve of a tube for circulating a cooling gas, and an ON/OFF operation of a heater for circulating forcibly cooling gas must be performed. Therefore, this method cannot respond to a demand for simplifying magnetizing and demagnetizing operations.
On the other hand, in current leads, in order to reduce an amount of thermal penetration to a very-low-temperature portion, a structure in which a liquid nitrogen anchor portion is arranged midway along a path from a room-temperature portion to the very-low-temperature portion is often employed. This method is effectively used for gas cooling type current leads for cooling a conductor by helium gas obtained by evaporating liquid helium for cooling a superconducting magnet. More particularly, the method is effectively used for reducing the amount of thermal penetration of current leads in which helium gas does not flow in an OFF time. FIG. 1 is a schematic view showing a conventional gas cooling type current lead having the liquid nitrogen anchor. Referring to FIG. 1, a current lead 1 has a cooling tube 2, a conductor 3 formed in the cooling tube 2, and a liquid nitrogen anchor portion 5. A cooling helium gas path 4 is formed between the conductor 3 and the cooling tube 2, and helium vapor is circulated in the path 4 to cool the conductor 3. In the liquid nitrogen anchor portion 5, a liquid nitrogen tube 7 is connected to the conductor 3 through an electric insulator 6, and the conductor 3 is cooled by liquid nitrogen circulated in the tube 7. The A and B sides of a main body 1 are connected to a room-temperature portion and a very-low-temperature portion, respectively, and the cooling helium flows in the path 3 from the B side to the A side.
However, the above current lead has the following problem. That is, cooling helium gas evaporated from a liquid helium tank on the very-low-temperature side exchanges heat with the current lead which generates heat, and the temperature of the helium gas is increased from 4.2 K. When the helium gas reaches the liquid nitrogen anchor portion 5, the temperature of the helium gas may be lower than the freezing point of nitrogen of 63.3 K (at 1 atmospheric pressure). In this case, liquid nitrogen is frozen in the tube 7 to clog the liquid nitrogen tube 7, thereby largely degrading reliability of the current lead.
As a means for solving the above problem, as shown in FIG. 2, a method in which a thermal switch 8 is arranged to the liquid nitrogen anchor portion 5 is proposed. Since the thermal switch 8 is turned on at a temperature of 77 K or more and turned off at the temperature of less than 77 K, when helium gas having a temperature of less than 66.3 K flows, the liquid nitrogen is not frozen because the liquid nitrogen tube 7 is thermally insulated from the main body 1 of the current lead.
In the above technique, however, the structure of the current lead is complicated and large in size. In addition, when a gravity heat pipe described in a paper of the Advance Cryogenic Engineering Vol. 29 (1984) p. 658 by J. Yamamoto is used as the thermal switch, a location in use of a current lead is restricted due to the gravity dependency of the gravity heat pipe.